Effect of humidity on microwave-based direct air capture under fluidization

This study explores the effect of humidity on CO₂ desorption from a microwave-assisted direct air capture system using Zeolite 13X as a sorbent. Relative humidity of air varied from 25 % to 75 % at room temperature in adsorption. The experiments were performed to investigate potential impacts of water molecules in air, where several adsorption paths (path 1, path 2, and path 3) were developed to mitigate these effects. Adsorption time varied between 30 and 100 min for the proposed adsorption paths. In desorption, a mono-mode microwave unit was used due to its ability of rapid and volumetric heating as well as creating homogeneous temperature distribution. Temperature in desorption was changed from 33 to 100 °C, while microwave initial power was set to either 4 W or 8 W depending on the target temperature. Desorption time varied between 10 and 33 min based on the temperature and microwave initial power. Both adsorption and desorption stages were successfully completed under various relative humidity, temperature, and microwave initial power conditions. This study presented water adsorption and desorption under varying temperature and humidity conditions, with particular emphasis on CO₂ purity, productivity, and energy demand of microwave-assisted fluidized bed regeneration. The findings illustrated that roll-up effect in adsorption can be mitigated by selecting appropriate adsorption pathways, thereby enhancing the overall performance of hydrophilic sorbents in humid environments. Furthermore, cyclic adsorption-desorption experiments revealed that Zeolite 13X exhibited a significant deterioration, exceeding 90 % under repeated cycles.